Concentrations of Credit Risk - Commercial Real Estate Loans (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Concentration Risk [Line Items]
Total Loans	$ 43,390.6	$ 47,617.0
Commercial
Concentration Risk [Line Items]
Total Loans	20,278.8	25,412.8
Commercial Real Estate | Commercial
Concentration Risk [Line Items]
Total Loans	5,314.2	5,134.2
Total Commercial Mortgages | Commercial
Concentration Risk [Line Items]
Total Loans	4,746.3	4,552.3
Office | Commercial
Concentration Risk [Line Items]
Total Loans	944.4	1,035.1
Apartment/ Multi-family | Commercial
Concentration Risk [Line Items]
Total Loans	1,599.9	1,633.9
Retail | Commercial
Concentration Risk [Line Items]
Total Loans	665.6	620.9
Industrial/ Warehouse | Commercial
Concentration Risk [Line Items]
Total Loans	906.1	687.1
Other | Commercial
Concentration Risk [Line Items]
Total Loans	630.3	575.3
Construction, Acquisition and Development Loans | Commercial
Concentration Risk [Line Items]
Total Loans	$ 567.9	$ 581.9